Bring ¾ cup rice vinegar and 5 thyme sprigs to a boil in a small saucepan. Remove from heat and pour over 1 cup beets in a shallow heatproof bowl. Let marinate at room temperature, stirring occasionally, for 30 minutes. Drain the beets and pat dry.
Mash rinsed beans, ½ teaspoon lemon zest, 2 tablespoons each lemon juice and oil, 1 teaspoon mustard, ½ teaspoon each coriander and crushed red pepper and ¼ teaspoon salt in a medium bowl until well combined but still slightly chunky. Stir in ¼ cup herbs.
Spread the bean mixture on one side of each of 4 toast slices. Top 2 slices with ½ cup pickled beets, ½ cup sprouts and 2 tablespoons red onion. Drizzle the vegetables with the remaining 1½ teaspoons lemon juice. Top with the remaining 2 toast slices, spread-side down. Cut in half and serve.
